1. Field of the Invention
The invention relates to a glass and a glass ceramic.
2. Description of Related Art
It is generally known that glasses of the Li2O—Al2O3—SiO2 system can be converted into glass ceramics having high quartz mixed crystals/solid solution and/or keatite mixed crystals/solid solution as primary crystal phases. The ceramicizing processes known for this purpose are described in a large number of publications. These involve a temperature process, by means of which the initial glass is converted into the glass-ceramic article by controlled crystallization. This so-called ceramicizing takes place in a two-step temperature process, as is known, in which typically first, nuclei are produced by an isothermal nucleation at a temperature between 680° C. and 810° C., usually from ZrO2/TiO2 mixed crystals. SnO2 can also participate in the nucleation.
With subsequent increase in temperature, the high quartz mixed crystals grow on these nuclei. High rates of crystal growth, such as are desired for an economical, rapid ceramicizing, are obtained at temperatures of 800° C. to 950° C., in each case depending on material composition. Typical for a rapid ceramicizing, i.e., a short ceramicizing duration, are high maximum temperatures. The latter usually lie above 880° C. and assure that a significant crystal growth can occur despite a short residence time in this range.
At the maximum temperature, the structure of the glass ceramic is homogenized and the optical, physical, and chemical properties are finely adjusted. If desired, the high quartz mixed crystals can subsequently still be converted into keatite mixed crystals. The conversion into keatite mixed crystals is produced with an increase in temperature in a temperature range of approximately 970 to 1250° C. With the conversion, the thermal expansion coefficient of the glass ceramic increases and with further crystal growth, light scattering occurs, combined with a translucent to opaque appearance.
A number of requirements are placed on glass ceramics and the production methods belonging thereto, particularly on the ceramicizing method, environmental friendliness gaining increasing significance.
Environmental friendliness is based on the fact that the glass ceramic is technically free of the usual refining agents, arsenic oxide and antimony oxide. As an impurity, these components are usually present in contents of less than 500 ppm, generally less than 200 ppm. In exceptional cases, the As2O3 content can be a maximum of 1000 ppm, if shards of a transparent glass ceramic containing arsenic oxide are added to the melt as a refining agent. Since an effective contribution to environmental protection is provided with such recycling due to savings in energy and raw materials, in this case, a higher As2O3 content of up to 1000 ppm is permissible.
The favorable manufacturing properties include low melting and shaping temperatures, resistance to devitrification, and rapid ceramicizing capability.
For economical ceramicizing, short ceramicizing times and low maximum temperatures, i.e., low energy requirements, are desired overall, whereby the transparency of the glass ceramic must not suffer due to coloring and scattering.
With applications of transparent glass ceramics, usually a high transparency, i.e., light transmittance (brightness Y) in the visible region greater than 84%, and little color (chromaticity) C*, thus a neutral hue, are desired.
In addition, visually disruptive light scattering that is made noticeable as clouding (haze) must not occur.
Absorption and scattering are thus the optical phenomena that must be mastered for economical production.
Due to the use of arsenic oxide as a refining agent, the requirements for a particularly high transparency (high light transmittance and slight color) and particularly favorable conversion kinetics of the glass ceramics are fulfilled in fact, but not the requirements for environmental friendliness.
The brownish coloring of transparent lithium aluminum silicate glass ceramics has different causes that are primarily based on absorption mechanisms and on scattering.
The requirement for environmental friendliness is fulfilled, e.g., by the use of SnO2 instead of As2O3 as an environmentally-friendly refining agent, but of course, the emergent Sn/Ti complexes bring about an additional absorption to Fe/Ti color complexes.
The coloring element Fe is contained as an impurity in the batch raw materials for the melting. The latter colors ionically as Fe3+ as well as via Fe/Ti color complexes. Due to the high cost of low-iron raw materials, it is not economical to reduce the Fe2O3 content to values of 100 ppm and thereunder.
Electronic transitions in color complexes, which absorb in the short-wave region of visible light and in which participates the TiO2 component that is effective for the nucleation, make up the most intense absorption mechanism of transparent glass ceramics. A color complex arises due to the formation of adjacent Fe and Ti ions, between which occur electronic charge-transfer transitions.
The Fe/Ti color complexes lead to a red-brown coloring; the Sn/Ti color complexes lead to a yellow-brown coloring. The Sn/Ti color complexes color more intensely and this circumstance has previously made it difficult to substitute the refining agent arsenic oxide by SnO2 in the case of transparent glass ceramics. The formation of the named color complexes occurs significantly during ceramicizing.
The scattering in transparent glass ceramics is largely determined by the crystallite size and the different indices of refraction of the high quartz mixed crystals and the residual glassy phase, and thus also is largely determined by the ceramicizing. For minimizing the light scattering, it is necessary to align the refraction indices in the region of visible light. The crystallite size should be clearly smaller than the wavelength of visible light and the birefringence of the crystals should be small (Sakamoto et al. “Structural relaxation and optical properties in nanocrystalline β-quartz glass-ceramic”, Journal of Non-Crystalline Solids 352 (2006) pp. 514-518).
Small crystallite sizes are obtained with a high density of nuclei and a low maximum temperature for the entire ceramicizing, which lead to the circumstance that the size of the growing high quartz mixed crystals lie below the wavelength of visible light. Typically, the average crystallite size of the high quartz mixed crystals lies in the range of 20 to 60 nm. A high nuclei density presupposes a sufficient content of nucleating agents as well as sufficient nucleation times as well as kinetic properties of the initial glass during the ceramicizing.
For the aligning of the refraction indices in the region of visible light, it is necessary to find favorable composition ranges and ceramicizing conditions, since the composition of the high quartz mixed crystals and the residual glassy phase is a consequence of composition as well as ceramicizing conditions. In this case, the surface layers, and not only the intermediate regions between the individual crystallites within the bulk structure, are included in the so-called residual glassy phase. Consequently, the differences in the refractive indices between surface layer and bulk structure are also decisive for scattering in the transparent, low-color glass ceramic.
The effective nucleating agent TiO2 can only be substituted with disadvantages in melting and shaping by the alternative nucleating agents ZrO2 and SnO2. This means that the desired low melting points and short ceramicizing times lead to an intensified coloring based on the color complexes without visually disruptive scattering via the TiO2 contents required therefor.
Numerous developmental attempts have been made for producing environmentally-friendly, transparent glass ceramics without the use of the refining agents, arsenic oxide and antimony oxide. These could not be implemented previously for technical and economic reasons. Transparency, i.e., high light transmittance and slight coloring without visually disruptive scattering, could not be reconciled with favorable manufacturing conditions.
One approach involves compositions without the nucleating agent TiO2, which lead to disadvantages during production.
Thus, WO 2008/065167 A1 describes the production of environmentally-friendly, transparent glass ceramics without disruptive coloring. These glass ceramics avoid the addition of TiO2 as a nucleating agent and are based on a mixed nucleation by ZrO2 and SnO2. The ZrO2 contents necessary for a sufficiently rapid nucleation are 2-5 wt. %, and the necessary SnO2 contents are >0.4-3 wt. %. With these high contents of ZrO2 and SnO2 the melting of the batch is slowed down, the melting and shaping temperatures are increased, and the resistance to devitrification of the glass is adversely affected. During the shaping, which takes place at viscosities of 104 dPas around the processing temperature VA, disruptive crystal phases containing Sn and Zr crystallize out. This leads to the circumstance that the following ceramicizing process can almost no longer be controlled via the temperature control. In addition, such compositions require extraordinarily high ceramicizing temperatures and thus represent an uneconomical method.
Another approach involves transparent glass ceramics without arsenic oxide and antimony oxide as refining agents and with small contents of TiO2, but which also require higher contents of SnO2 and ZrO2 as nucleating agents. In WO 2008/065166 A1, TiO2 is limited to 0.3-<1.6 wt. %. Contents of SnO2 from 0.25-1.2 wt. % and ZrO2 from >2-3.8 wt. % are required. These high contents are accompanied by the described disadvantages in melting and shaping as well a deficient resistance to devitrification.
The documents JP 11-228180 A2 and JP 11-228181 A2 describe environmentally-friendly compositions of transparent glass ceramics. In order to obtain sufficient bubble qualities without using arsenic oxide as a refining agent, the glass ceramic contains a combination of the refining agents SnO2 and Cl of 0.1-2 wt. %. The physical decoloring agent Nd2O3 is not used, so that the Sn/Ti color complex fully comes into play. In particular, the high SnO2 contents displayed in the embodiment examples are very harmful for the resistance to devitrification. The documents do not provide any hints of how the SnO2 content must be limited so as to assure sufficient resistance to devitrification. In addition, these publications supply no hint of an optimization of the manufacturing properties by the selection of the components CaO and SrO and the adjustment of crystal composition and composition of the residual glassy phase by the ratios of the divalent components MgO, ZnO, as well as CaO, SrO and BaO.
The physical decoloring of transparent glass ceramics by additions of Nd2O3 and CoO, which absorb in the longer-wave red spectral region is disclosed in EP 1 837 312. The document preferably describes compositions refined with arsenic oxide. In addition to the use of arsenic oxide, the use of 0.1-0.4 wt. % SnO2 in combination with high-temperature refining above 1700° C. is also disclosed as an environmentally-friendly refining agent. This document does not provide any hint as to how the composition must be created in order to obtain particularly favorable manufacturing conditions, i.e., low melting and low shaping temperatures. There is thus a need for decreasing the melting and shaping temperatures without disadvantages for the rate of ceramicizing, since this is of crucial importance for energy efficiency and economical production.
Common to all of these cited documents is that they describe a two-step ceramicizing process.
The ceramicizing programs that are described in EP 1 837 312 and EP 1 837 314 and are characterized by processing times of less than 2 hours, after the necessary heating phases, have ceramicizing steps with holding times, e.g., at 790° C. and 900° C., on the order of magnitude of 5-30 minutes. The known ceramicizing methods essentially involve isothermal ceramicizing methods.
It has been shown that elaborate experiments are required for each glass composition in order to fine-tune the method parameters to one another so that the glass ceramic has the desired properties, in particular with respect to color and scattering. The experiments are time-consuming and costly.